Some problems are too big to be handled alone. AIDS in India is one of them.

In a country of a billion people, about 4.6 million are HIV positive. If the problem is left unchecked, that number could reach 20 million to 25 million by the decade’s end. A single country could have an HIV-positive population larger than the total populations of London, New York, and Tokyo combined. Ashok Alexander is the director of Avahan (Sanskrit for "call to action"), the India AIDS initiative launched in April 2003 by the Bill & Melinda Gates Foundation. He believes that India’s epidemic can be stopped before it approaches the proportions seen today in sub-Saharan Africa—but only by building a vast network of public-private alliances on a scale rarely attempted. With each partner bringing distinct skills and assets to bear on the crisis, careful coordination is essential.

Conditions in India could promote the rapid spread of AIDS in coming years. Although among adults its prevalence¹ is only 0.8 percent—compared with almost 39 percent in Botswana and 33 percent in Zimbabwe, the two most heavily stricken countries—overpopulation and widespread poverty are already straining the government’s resources. The public-health infrastructure, facing a variety...